Burst dimming cycles can be used to control brightness of a light source, e.g., a light emitting diode (LED). A burst dimming cycle includes an ON period and an OFF period. A plurality of current pulses pass through the light source during the ON period and no current flows through the light source during the OFF period. Thus, the brightness of the light source can be controlled by adjusting duty cycle of the burst dimming cycles.
FIG. 1(a) shows the waveform of a burst dimming signal 110 for controlling the brightness of a light source. The burst dimming signal 110 is switched between an ON period and an OFF period alternately. The durations of the ON period and the off period can be predetermined. FIG. 1(b) shows an average current flowing through the light source controlled by the burst dimming signal 110 under an ideal circumstance. Thus, the average current of the light source is substantially constant during an ON period of the burst dimming signal 110 and is zero during an OFF period of the burst dimming signal 110. However, in practical applications, a capacitor may be coupled to the light source in parallel. During the OFF period, the capacitor is discharged to the light source and thus a voltage of the capacitor drops to zero quickly. During the ON period, the voltage of the capacitor gradually rises and no current flows through the light source until the voltage of the capacitor rises to a certain level. Thus, there is a startup phase of the current of the light source. FIG. 1(c) shows an average current flowing through the light source controlled by the burst dimming signal 110 in a practical application. As shown in FIG. 1(c), the average current of the light source gradually increases from zero. During the startup phase, almost no current flows through the light source. The duration of the startup phase varies in different practical applications. Therefore, the time period when the average current of the light source is substantially constant during an ON period of the burst dimming signal is uncertain and varies in different applications. As a result, the brightness of the light source is not controlled very accurately and the brightness of the light source may vary in different applications.
FIG. 2 shows a burst dimming driving circuit 200 in the prior art. A converter formed by an inductor 202, a diode 204, and a switch 206 converts an input voltage VIN to an output voltage VOUT to power a light source, e.g., an LED string 230, and produce a current through the LED string 230. The driving circuit 200 further includes a switch 220. A capacitor 240 is coupled to the LED string 230 and the switch 220 in parallel. The switch 220 is controlled by a burst dimming signal at a pin PWMOUT of a controller 210. A pulse-width modulation (PWM) signal is received by a pin PWM of the controller 210. The burst dimming signal having an ON period and an OFF period is generated at the pin PWMOUT according to the PWM signal. During the OFF period, the switch 220 is turned off to disconnect the LED string 230 from the capacitor 240. Thus, the voltage of the capacitor 240 drops in a relatively slow speed. When the ON period starts, the switch 220 is turned on and the voltage of the capacitor 240 is still beyond a certain level. Thus, the current through the LED string 230 can be established faster compared to the prior art in FIG. 1. Therefore, the accuracy of the ON period is improved, thereby enhancing the accuracy of the dimming control. However, the cost of the burst dimming driving circuit 200 is relatively high because of the extra pins PWM and PWMOUT and the switch 220.